That's normal. cdrecord uses a kernel function which is only
available for root (to better avoid buffer underruns).
If it was available for all users this would by a security leck...
Because this function can be used to crack the system if you
use it intelligent...

If you want to burn as normal user to it like man cdrecord describes:

- Create new group burn
- All users allowing to burn write into this group
- chmod root.burn /usr/bin/cdrecord
- chmod 4710 /usr/bin/cdrecord

This was it. Log out, log in and you'll be able to burn...

>
> If I su to root and issue the same command, the usual startup-messages
> appear, but no real burning takes place when it should start. I just see
> this message:
> Waiting for reader process to fill input buffer...
> And the source-drive spins up and down forever.

.. if that won't happen. I do not know how to help here...
>
> Burning from ISO-images to the CD-RW drive works fine.